1. Field
Embodiments of the present invention relate to a lithographic apparatus and method for acquiring height data of a substrate surface, to a program and a memory containing the program for acquiring height data and to a method, apparatus, program and memory for correcting height data acquired according to said method.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. This is done using a projection system that is between the reticle and the substrate and is provided to image an irradiated portion of the reticle onto a target portion of a substrate. The pattern can be imaged onto an exposure area (die) on the substrate (silicon wafer). In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). This is described in more detail below.
In current dual stage apparatus, data is gathered to level every target portion (field) with a level sensor in exactly the same position with respect to the center of the target portion.
The projection system includes components to direct, shape and/or control a beam of radiation. The pattern can be imaged onto the target portion (e.g., including part of one, or several, dies) on a substrate, for example a silicon wafer, that has a layer of radiation-sensitive material, such as resist. In general, a single substrate contains a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction, usually referred to as the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
A lithographic apparatus can contain a single mask table and a single substrate table, but are also available having at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791, which are incorporated herein in their entireties. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
During exposure processes, it is important to ensure that the mask image is correctly focused on the wafer. Conventionally this has been done by measuring the vertical position of the best focal plane of the aerial image of the mask relative to the projection lens before an exposure or a series of exposures. During each exposure, the vertical position of the upper surface of the wafer relative to the projection lens is measured and the position of the wafer table is adjusted so that the wafer surface lies in the best focal plane.
Referring to FIG. 1, the scope for adjusting the position of the focal plane of the projection system PL is limited and the depth of focus of that system is small. This means that the exposure area of the wafer (substrate) must be positioned precisely in the focal plane of the projection system PL.
Wafers are polished to a very high degree of flatness but nevertheless deviation of the wafer surface from perfect flatness (referred to as “unflatness”) of sufficient magnitude noticeably to affect focus accuracy can occur. Unflatness may be caused, for example, by variations in wafer thickness, distortion of the shape of the wafer or contaminants on the wafer holder. The presence of structures due to previous process steps also significantly affects the wafer height (flatness). In the present invention, the cause of unflatness is largely irrelevant; only the height of the top surface of the wafer is considered. Unless the context otherwise requires, references below to “the wafer surface” refer to the top surface of the wafer onto which will be projected the mask image.
During exposures, the position and orientation of the wafer surface relative to the projection optics are measured and the vertical position (Z) and horizontal tilts (Rx, Ry) of the wafer table WT are adjusted to keep the wafer surface at the optimal focus position.
As described above, imaging a pattern onto a substrate W is usually done with optical elements, such as lenses or mirrors. In order to generate a sharp image, a layer of resist on the substrate W should be in or near the focal plane of the optical elements. Therefore, according to the prior art, the height of the target portion C that is to be exposed is measured. Based on these measurements, the height of the substrate W with respect to the optical elements is adjusted, e.g., by moving the substrate table WT on which the substrate W is positioned. Since a substrate W is not a perfectly flat object, it may not be possible to position the layer of resist exactly in the focal plane of the optics for the whole target portion C, so the substrate W may only be positioned as well as possible.
In order to position the substrate W in the focal plane as well as possible (e.g., by matching the focal plane to the centre of the resist thickness), the orientation of the substrate W can be altered. The substrate table WT may be translated, rotated or tilted, in all six degrees of freedom, in order to position the layer of resist in the focal plane as well as possible.
In order to determine the best positioning of the substrate W with respect to the optical elements, the surface of the substrate W may be measured using a level sensor, as for instance described in U.S. Pat. No. 5,191,200, incorporated herein by reference in its entirety. This procedure may be done during exposure (on-the-fly), by measuring the part of the substrate W that is being exposed or is next to be exposed, but the surface of the substrate W may also be measured in advance. This latter approach may also be done at a remote position. In the latter case, the results of the level sensor measurements may be stored in the form of a so-called height map or height profile and used during exposure to position the substrate W with respect to the focal plane of the optical elements.
In both cases, the top surface of the substrate W may be measured with a level sensor that determines the height of a certain area. This area may have a width about equal to or greater than the width of the target portion C and may have a length that is only part of the length of target portion C, which will be explained below (the area being indicated with the dashed line). The height map of a target portion C may be measured by scanning the target portion C in the direction of the arrow A. The level sensor LS determines the height of the substrate W by applying a multi-spot measurement, such as for instance a 9-spot measurement. Level sensor spots LSS are spread over the area and, based on the measurements obtained from the different level sensor spots, height data may be collected.
The term “height” as used here refers to a direction substantially perpendicular to the surface of the substrate W, i.e., substantially perpendicular to the surface of the substrate W that is to be exposed. The measurements of a level sensor result in height data, including information about the relative heights of specific positions of the substrate W. This may also be referred to as a height map.
Based on this height data, a height profile may be computed, for instance by averaging corresponding height data from different parts of the substrate (e.g., height data corresponding to similar relative positions within different target portions C). In case such corresponding height data is not available, the height profile is equated to the height data.
Based on height data or a height profile, a leveling profile may be determined to provide an indication of an optimal positioning of the substrate W with respect to a projection system PS. Such a leveling profile may be determined by applying a linear fit through (part of) the height data or the height profile, e.g., by performing a least squares fit (three dimensional) through the points that are inside the measured area.
As explained above, accurate leveling may require measuring the shape and topography of the substrate, for instance using a level sensor, resulting in height data of (at least part) of the substrate W, based on which a leveling profile can be determined. Such a leveling profile may represent the optimal position of the substrate W with respect to the projection system PS, taking into account the local shape and height of the substrate W.
By leveling every target portion in exactly the same position with respect to the centre of the target portion, variation of exposure focus between target portions on a substrate is reduced.
For example, every target portion may have a leveling sampling pattern with respect to the center of the target portion as shown in FIG. 2. In FIG. 2, M2-M6 refer to the level sensor spots, covering the target portion widths, which are scanned over the surface over the target portion along the target portion height. During the scanning, a number of measurements are gathered, indicated by the dots in FIG. 2. These dots have a predetermined position with respect to the target portion center.
It is relatively time consuming to take leveling data according to the technique as shown and described with reference to FIG. 2. This is because it is time consuming to take leveling data for each target portion that meets the constraint of positioning the level sensor in the same position with respect to the center of each target portion. In the most common case of a columnar target portion layout, this requires a ‘stroke’ of level sensor readings through each column.
This is further illustrated with reference to FIG. 4, showing a substrate with a plurality of target portions and arrows indicating the scanning path or strokes of the level sensor.